U.S. patent number 8,320,287 [Application Number 12/407,958] was granted by the patent office on 2012-11-27 for method and system for the indication of long drx in a wireless network.
This patent grant is currently assigned to Research In Motion Limited. Invention is credited to Zhijun Cai, Takashi Suzuki, James Earl Womack, Gordon Peter Young.
United States Patent |
8,320,287 |
Cai , et al. |
November 27, 2012 |
Method and system for the indication of long DRX in a wireless
network
Abstract
Methods and apparatus for configuring a discontinuous reception
`DRX` period at from an evolved Node B for a user equipment,
including: sending a long DRX MAC CE from the eNB if preconditions
are met, and at the UE determining whether a short DRX period is
configured; if no, setting a long DRX period; if yes, determining
whether a long DRX command was received in a medium access control
`MAC` control element; if yes, setting the long DRX period; if no,
starting a short DRX timer and setting a short DRX period.
Inventors: |
Cai; Zhijun (Euless, TX),
Young; Gordon Peter (Slough, GB), Womack; James
Earl (Bedford, TX), Suzuki; Takashi (Tokyo,
JP) |
Assignee: |
Research In Motion Limited
(Waterloo, Ontario, CA)
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Family
ID: |
40719038 |
Appl.
No.: |
12/407,958 |
Filed: |
March 20, 2009 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090238098 A1 |
Sep 24, 2009 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61038432 |
Mar 21, 2008 |
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Current U.S.
Class: |
370/311;
370/310 |
Current CPC
Class: |
H04W
76/28 (20180201) |
Current International
Class: |
H04B
7/00 (20060101); G08C 17/00 (20060101) |
Field of
Search: |
;370/311,310
;455/574 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Ericsson, "Details of MAC DRX Control," TSG-RAN WG2 Meeting #61,
Sorento, Italy, Feb. 11-15, 2008, (R2-080934), XP-002532356. cited
by other .
3GPP TS 36.321 v8.1.0 (Mar. 2008), "3rd Generation Partnership
Project; Technical Specification Group Radio Access Network;
Evolved Universal Terrestrial Radio Access (E-UTRA), Medium Access
Control (MAC) protocol specification (Release 8)". cited by other
.
Research in Motion Limited, "Go to Long Sleep Command for LTE DRX,"
3GPP TSG-RAN-WG2 Meeting #61bis, Shenzhan, China, Mar. 31-Apr. 4,
2008 (R2-081868), XP-002532357. cited by other .
PCT/US2009/037760, International Search Report dated Jul. 2, 2009.
cited by other .
PCT/US2009/037760, Written Opinion dated Jul. 2, 2009. cited by
other .
International Preliminary Report on Patentability for
PCT/US2009/037760, dated Sep. 30, 2010 (11 pages). cited by other
.
Change Request, 3GPP TSG-RAN2 Meeting #62bis, Warsaw, Poland, Jun.
30-Jul. 4, 2008 (4 pages). cited by other .
International Preliminary Report on Patentability for
PCT/US2009/041769, dated Aug. 18, 2010 (14 pages). cited by other
.
Ericsson, Clarification of DRX, R2-083895, 3GPP TSG-RAN2 Meeting
#62bis, Jul. 4, 2008, Warsaw, Poland (4 pages). cited by
other.
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Primary Examiner: Sheikh; Ayaz
Assistant Examiner: Hampton; Tarell
Attorney, Agent or Firm: Finnegan, Henderson, Farabow,
Garrett & Dunner, LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
The present application claims priority from U.S. Provisional
Application Ser. No. 61/038,432, filed Mar. 21, 2008, the contents
of which are incorporated herein by reference.
Claims
We claim:
1. A method for configuring a discontinuous reception `DRX` period
at a user equipment comprising: determining whether a short DRX
period is configured at the user equipment; when no short DRX
period is configured, setting a long DRX period; when the short DRX
period is configured, determining whether a long DRX command was
received in a medium access control `MAC` control element; when a
long DRX command is received in the MAC control element, setting a
long DRX period and delaying activation of the long DRX period in
accordance with a timing margin configured to synchronize the user
equipment DRX configuration with an evolved node B `eNB`; when a
long DRX command is not received in the MAC control element,
starting a short DRX timer and setting a short DRX period.
2. The method of claim 1, wherein the user equipment maintains a
short DRX configuration.
3. The method of claim 1, wherein the DRX is configured for non
real time traffic.
4. The method of claim 1, further comprising the step of stopping a
short DRX timer if the long DRX command in the MAC control element
was received.
5. A user equipment adapted to configure a discontinuous reception
`DRX` period, comprising: a communications subsystem, said
communications subsystem adapted to receive a medium access control
`MAC` control element; a memory, said memory adapted to store
information for the medium access control element; and a processor,
said processor adapted to: determine whether a short DRX period is
configured at the user equipment; when no short DRX period is
configured, set a long DRX period; when the short DRX period is
configured, determine whether a long DRX command was received in
the medium access control `MAC` control element; when a long DRX
command is received in the MAC control element, set a long DRX
period and delay activation of the long DRX period in accordance
with a timing margin configured to synchronize the user equipment
DRX configuration with an evolved node B `eNB`; when a long DRX
command is not received in the MAC control element, start a short
DRX timer and set a short DRX period.
6. The user equipment of claim 5, wherein the memory is adapted to
maintain a short DRX configuration.
7. The user equipment of claim 5, wherein the DRX is configured for
non real time traffic.
8. The user equipment of claim 5, wherein the processor is further
adapted to stop a short DRX timer if the long DRX command in the
MAC control element was received.
9. The method of claim 1, wherein timing margin is derived from a
header of the MAC control element.
10. The method of claim 1, wherein the DRX configuration comprises
DRX activation and DRX deactivation.
11. The method of claim 1, further comprising: determining, by the
user equipment, the timing margin.
12. The user equipment of claim 5, wherein timing margin is derived
from a header of the MAC control element.
13. The user equipment of claim 5, wherein the DRX configuration
comprises DRX activation and DRX deactivation.
14. The user equipment of claim 5, wherein the processor is further
configured to determining the timing margin.
Description
FIELD OF THE DISCLOSURE
The present disclosure relates to the long term evolution (E-UTRA)
of Third Generation Partnership Project (3GPP), and in particular
to discontinuous reception (DRX) for user equipment (UE) in the
E-UTRA infrastructure.
BACKGROUND
In the long term evolution infrastructure, a UE can be in one of
two radio resource control (RRC) states. These are LTE_IDLE and
LTE_ACTIVE.
The UE can be configured for discontinuous reception (DRX) in both
the LTE_IDLE and the LTE_ACTIVE states. DRX allows the UE to
synchronize its listening period to a known paging cycle of the
network. By synchronizing the listening period, the UE can turn off
its radio transceiver during the standby, thereby significantly
saving battery resources. As will be appreciated by those skilled
in the art, unless a UE is used extensively, a large drain on its
battery comes from the standby cycle in which it monitors the
paging channel and measures serving and neighboring cells. DRX
parameters allow the mobile to synchronize with the network and to
know that it will not receive another signal until a specified time
has elapsed.
Utilizing DRX in an IDLE state is performed in present UMTS systems
and is done by the network signaling to the UE a DRX parameter and
synchronizing the UE and the network. As will be appreciated, in
IDLE mode the UE can change cells from one cell to the other. Thus
utilizing a DRX parameter does not cause significant issues.
In an ACTIVE state however, various issues exist for turning off
the receiver based on a DRX parameter. This includes the fact that
only network controlled handover is allowed in the LTE_ACTIVE
state. Also, other issues include efficient signaling of activation
and deactivation of DRX, measurement requirements of network
signals during the DRX, handling of missed handover opportunities,
and issues dealing with the length of the DRX value in which entity
in the network can request DRX activation and reconfiguring the DRX
period.
In current systems, when short and long DRX is configured for non
real time traffic and the traffic volume becomes very low, the eNB
wants to put the UE which may be either in short DRX or actively
listening to downlink common control channels into long DRX, for
further battery saving if there is no uplink and downlink
transmission. In current E-UTRA specifications, the eNB needs to
deactivate short DRX using a RRC reconfiguration message at first
and then issue a go-to-sleep MAC control element (CE). If traffic
pattern changes, however, the eNB needs to activate short DRX by
RRC again.
BRIEF DESCRIPTION OF THE DRAWINGS
The present application will be better understood with reference to
the drawings, in which:
FIG. 1 is a block diagram showing a long term evolution user plane
protocol stack;
FIG. 2 is a block diagram showing a long term evolution control
plane protocol architecture;
FIG. 3a is a flow chart showing a method to activate deactivate and
reconfigure DRX period using a MAC-PDU header from the eNB
side;
FIG. 3b is a flow chart showing a method to acknowledge the
activation, deactivation or reconfiguration of the DRX period from
the UE side;
FIG. 4a is a flow chart showing a method for a UE to leverage
application traffic characteristics to improve battery life from
the UE side;
FIG. 4b is a flow chart showing a method for a UE to leverage
application traffic characteristics to improve battery life from
the eNB side;
FIG. 5 is a diagram showing signal strength thresholds and
measurement cycle times;
FIG. 6a is a flow chart illustrating procedural steps involved in
switching to a target eNB from the UE side;
FIG. 6b is a flow chart illustrating procedural steps involved in
switching to a target eNB from the eNB side;
FIG. 7 is a graph showing channel status going below a lower
threshold value and then above a threshold value without any uplink
data;
FIG. 8 is a graph showing channel status going below a lower
threshold value and then above a higher threshold value with uplink
data;
FIG. 9 is a graph showing signal degradation in which a handover
condition is triggered;
FIG. 10a is a flow chart showing a method to transition directly to
long DRX period using a MAC-PDU header from the eNB side; and
FIG. 10b is a flow chart showing a method to transition directly to
a long DRX period from the UE side.
DETAILED DESCRIPTION OF THE DRAWINGS
The present disclosure provides various methods and systems for
addressing the deficiencies in the prior art regarding DRX in an
LTE_ACTIVE state.
In particular, a DRX signaling procedure between the UE and the
eNB, in which the eNB signals DRX values and timing margins as part
of a modified MAC-PDU header, is described. The eNB signaled DRX
value can range from zero to indicate DRX deactivation to a value
for the DRX period. The timing margin can indicate a delay for
activating DRX to overcome NACK-ACK misinterpretations or ACK-NACK
misinterpretations. In one embodiment the timing margin can be also
signaled by the RRC.
In a further embodiment, the DRX value can be a relative value that
is predefined on the eNB or the UE or a pre-configured value by RRC
signaling. Thus a "short DRX value" could be used to indicate a
short DRX period and a "long DRX value" could be used to indicate a
long DRX period. In further embodiments, more than two values are
possible. In this case, the eNB can signal, for example, a long DRX
value in a "go-to-long-sleep" command to allow the UE to proceed
directly to long DRX without having to transition first to a short
DRX value and wait for the short DRX period timer to expire before
transitioning to a long DRX period, and without RRC reconfiguration
commands.
The DRX value, in one embodiment can be incrementally increased to
a certain maximum value that will be either defined in the
standards or signaled. The increment may be carried out without
signaling by both the UE and the eNB if no data has been received
for a preset number of DRX cycles. In a further embodiment, the DRX
value can be incrementally decremented until DRX is deactivated
without signaling by both the UE and the eNB.
In a further embodiment, application level traffic characteristics
can be leveraged for an optimization of the DRX period to improve
battery life. The UE could, in this case, send a request to
initiate or amend a DRX value to the eNB and the eNB can either
accept this value or reject it. Various considerations including
mobility, location of the cell, traffic characteristics, or missed
handover opportunities can be determined for both the UE and the
eNB in choosing and accepting a DRX value.
In a further embodiment, measurement accuracy may be improved by
shortening the measurement cycle from the DRX cycle if a certain
threshold signal value is reached for a certain amount of time.
Thus, in the case of signal degradation, the UE may decide that
more frequent measurement needs to be performed if the quality of
the signal falls below a threshold for a predetermined time.
Subsequently, the measurement cycle can be increased if the signal
rises above a threshold for a certain time period, or a handover
condition can be triggered if the signal falls below a
threshold.
In a further embodiment, missed handover opportunities can be
handled if the channel quality or signal strength of a serving cell
is less than a neighboring cell by a threshold value for a certain
duration. Procedures for switching to a target eNB are
disclosed.
The present disclosure therefore provides a method of DRX signaling
in a long-term evolution infrastructure between an evolved node B
(eNB) and user equipment (UE), the method comprising the steps of:
providing a DRX value or coded DRX value in a header of a medium
access control protocol data unit (MAC-PDU); and activating,
deactivating or reconfiguring DRX based on the provided DRX
value.
The present disclosure further provides a method of leveraging
application level traffic characteristics to improve battery life
of user equipment (UE) communicating with an evolved Node B (eNB)
comprising the steps of: requesting, from the UE, a discontinuous
reception (DRX) based on application traffic characteristics of the
UE; receiving the request from the UE at the eNB; and granting,
negotiating an alternative period or rejecting the request at the
eNB.
The present disclosure further provides a method for improved
measurement accuracy during discontinuous reception (DRX) on user
equipment (UE) comprising the steps of: checking whether a channel
quality or signal strength of a serving cell is lower than a first
threshold for a predetermined time period; and if yes, shortening
the measurement cycle to have a shorter measurement cycle than the
DRX cycle.
The present disclosure still further provides a method of handling
missed handover opportunities based on discontinuous reception
(DRX) in user equipment (UE) comprising the steps of: checking
whether a channel quality or signal strength of a serving cell is
less than the channel quality or signal strength of a neighboring
cell by a threshold value for a certain time duration; and if yes,
connecting to the neighboring cell.
The present disclosure further provides: an evolved node B (eNB)
operating in a long-term evolution infrastructure, the eNB being
characterized by means for providing a DRX value in a header of a
medium access control protocol data unit (MAC-PDU); and activating
or deactivating DRX based on the DRX value.
The present disclosure still further provides a user equipment (UE)
operating in a long-term evolution (LTE) infrastructure, the UE
being characterized by means for receiving a DRX value in a header
of a medium access control protocol data unit (MAC-PDU) and
acknowledging the MAC-PDU; and activating, deactivating or
reconfiguring DRX based on the DRX value.
Reference is now made to the drawings. FIG. 1 shows a block diagram
illustrating the long-term evolution (LTE) user plane protocol
stack.
A UE 110 communicates with both an evolved Node B (eNB) 120 and an
access gateway (aGW) 130.
Various layers are illustrated in the protocol stack. The packet
data convergence protocol (PDCP) layer 140 is illustrated both on
the UE 110 and on aGW 130. The PDCP layer 140 performs internet
protocol (IP) header compression and decompression, encryption of
user data, transfer of user data and maintenance of sequence
numbers (SN) for radio bearers.
Below the PDCP layer 140 is the radio link control protocol layer
142, which communicates with the radio link control protocol layer
142 on the eNB 120. As will be appreciated, communication occurs
through the physical layer in protocol stacks such as those
illustrated in FIGS. 1 and 2. However, RLC-PDUs from the RLC layer
142 of the UE are interpreted by the RLC layer 142 on the eNB
120.
Below RLC layer 142 is the medium access control (MAC) data
communication protocol layer 146. As will be appreciated by those
skilled in the art, the RLC and MAC protocols form the data link
sublayers of the LTE radio interface and reside on the eNB in LTE
and user equipment.
The layer 1 (L1) LTE (physical layer 148) is below the RLC/MAC
layers 142 and 146. This layer is the physical layer for
communications. Referring to FIG. 2, FIG. 2 illustrates the LTE
control plane protocol architecture. Similar reference numerals to
those used in FIG. 1 will be used in FIG. 2. Specifically, UE 110
communicates with eNB 120 and aGW 130. Further, physical layer 148,
MAC layer 146, RLC layer 142 and PDCP layer 140 exist within FIG.
2.
FIG. 2 also shows the non-access stratum (NAS) layer 210. As will
be appreciated, NAS layer 210 could include mobility management and
session management.
The radio resource control protocol layer (RRC) 220, is the part of
the protocol stack that is responsible for the assignment,
configuration and release of radio resources between the UE and the
E-UTRAN (Evolved universal terrestrial radio access network). The
basic functionalities of RRC protocol for LTE is described in 3GPP
TR25.813.
As will be appreciated by those skilled in the art, in UMTS,
automatic repeat request (ARQ) functionality is carried out within
the RLC layer which resides in the radio network controller (RNC).
Long Term Evolution (LTE) moves the ARQ functionality from the RNC
to eNB where a tighter interaction may exist between the ARQ and
the HARQ (within the MAC layer, also located in the eNB).
Various issues regarding DRX in an LTE-ACTIVE state are considered
herein.
DRX Signaling Procedure
Very efficient signaling procedures for activating and
de-activating DRX and specifying the duration of DRX periods are
required in order to support a large population of UEs in a cell
that are utilizing DRX in an LTE_ACTIVE state.
As will be appreciated by those skilled in the art, if the evolved
node B (eNB) transmits data to the UE during its receiver off
period due to a DRX operation, the UE cannot receive the data.
Therefore, an indication is required to ensure the UE and the eNB
are synchronized regarding when DRX is activated and
deactivated.
The indication between the UE and the eNB can be explicit signaling
by the radio resource control (RRC) or layer 1/layer 2 (L1/L2)
signaling. As will be appreciated, however, explicit signaling may
not be as efficient as desired.
A more efficient solution is to include an optional field in the
MAC header of a MAC-PDU (MAC Protocol Data Unit) to indicate DRX
activation and deactivation. The field preferably indicates the DRX
value and timing margin for activation and deactivation. A value of
zero, for example, could mean DRX deactivation in the DRX value
field in a preferred embodiment. Conversely, if data that is to be
transmitted in the next MAC-PDU is the last one in the buffer for
the UE, the eNB may extend the MAC header field to include a DRX
length initial value. For example, this could be 320 milliseconds.
The timing margin is explained below, and is utilized to reduce the
consequences of a NACK to ACK or ACK to NACK misinterpretation, for
the reception status of the MAC-PDU between the UE and the eNB.
For example, three bits may be added to the MAC header to indicate
eight values of the DRX period. Thus, rather than a specific time
value being sent, a bit value from 000 to 111 could indicate one of
eight discrete values.
In an alternative, a smaller field in the MAC header could be used
(for example two bits) to indicate increment or decrement. The RRC
could indicate default values, and if the MAC header indicates
increment or decrement then the UE could change to the prespecified
value.
Once the UE receives the DRX value, it acknowledges it to the eNB
by transmitting HARQ ACK and starts the DRX at the system frame
time considering propagation delay and processing delay at the eNB.
When the eNB receives the ACK from the UE, it also starts the DRX
at the next system frame time. As will be appreciated, the eNB does
not turn off its transceiver, but simply knows not to transmit
messages to the individual UE.
During a DRX period, if new data arrives at the eNB, the eNB can
send a MAC-PDU with a header extension set to DRX deactivation or a
shorter DRX length depending on the amount of data in the buffer or
the quality of service requirements. The UE reconfigures the DRX
accordingly and acknowledges the MAC-PDU. When the eNB receives the
ACK, it reconfigures the DRX. As indicated above, the deactivation
could be accomplished by merely setting the length value to
zero.
Reference is now made to FIGS. 3a and 3b. FIG. 3a shows an
exemplary method for controlling DRX activation in an LTE_ACTIVE
state. The process starts at step 300 and proceeds to step 310 in
which data is transmitted to the UE. As will be appreciated by
those skilled in the art, data transmission in an LTE_ACTIVE state
utilizes the MAC-PDU at the data link layer to transmit the
data.
The process next proceeds to step 312 in which a check is made to
see whether the buffer of data to be sent to the UE will be empty
after the next transmit. If no, the process proceeds back to step
310 in which data is transmitted to the UE. Alternatively, if the
buffer will be empty after the next transmit and the data arrival
rate is lower than a threshold value, the process proceeds to step
314.
In step 314, the eNB sets DRX activation in the MAC-PDU header. As
indicated above, this includes a DRX activation value indicating
the length of the DRX period. In another embodiment the eNB may
simply indicate an increase in the DRX interval. The UE
reconfigures the existing DRX interval to a predetermined reduced
interval. The predetermined interval may be either known to both
eNB and UE or pre-signaled to the UE from the eNB via explicit
signaling; either by system broadcast or RRC signaling.
The process then proceeds to step 316 in which the data including
the modified MAC-PDU header is sent to the UE.
Reference is now made to FIG. 3b. In step 318, the UE receives the
data and sees that DRX activation is specified in the MAC-PDU
header. The process proceeds to step 320 in which the UE sends an
acknowledgement (ACK) to the eNB and starts the DRX at the system
frame time considering propagation delay and processing delay at
the eNB.
In step 330 of FIG. 3a, the eNB receives the ACK from the UE and
starts the DRX at the next system frame time.
As will be appreciated, the DRX can continue until various events
occur which may require the DRX to be adjusted. One event is the
reception of data from aGW by the eNB for the UE. Depending on the
amount of data received, the DRX can either be deactivated or the
period of the DRX can be reduced. Other events that may require the
adjustment of the DRX include a change of signal power level
between the eNB and the UE or possibly a gradual increase in the
DRX cycle due to continued data inactivity, among others. These
other events are discussed in more detail below.
In step 332 the eNB checks to see whether the DRX needs to be
adjusted. As indicated above, this could be the situation where
data is received to be sent to the UE. Here the DRX can either be
deactivated or the period adjusted.
From step 332, if the DRX does not need to be adjusted, the process
proceeds back to step 332 and continues to check whether or not the
DRX needs to be adjusted.
Once the process in step 332 finds that the DRX does need to be
adjusted, the process proceeds to step 334 in which it adjusts the
DRX. This could be deactivating the DRX by transmitting a zero
value for the DRX or a shorter DRX or a longer DRX as required.
The MAC-PDU with the modified header is sent to the UE in step 336.
The MAC-PDU in step 336 could also include any data that has been
received by the eNB that needs to be transmitted to the UE.
Referring to FIG. 3b, the process then proceeds to step 318 in
which the MAC-PDU with modified header is received at the UE. The
UE recognizes the DRX period is to be adjusted and in step 320 it
sends an acknowledgement to the eNB and it adjusts its DRX period
at the same system frame time considering propagation delay and
processing delay as at the eNB.
Referring to FIG. 3a, in step 342 the eNB receives the ACK and
starts the modified DRX period at the appropriate system frame
time. The process then proceeds back to step 332 to see whether the
DRX needs to be adjusted again.
In one embodiment, a DRX command MAC control element could indicate
to a UE to transition to a DRX period. In this case, if the eNB
wants the UE to transition to a long DRX period due to lack of
uplink and downlink traffic and based on low traffic rates for non
real time DRX, under current E-UTRA specifications this requires a
change in the DRX configuration to be made with an RRC
configuration message. A problem with this is that if the eNB later
receives traffic patterns that require a shorter DRX period, the
RRC configuration message will need to be sent again to reconfigure
the DRX configuration on the UE.
Instead, a MAC CE could include a "go-to-long-sleep" possibility.
Thus, the eNB could provide the UE with an option to go directly to
a long DRX period without a reconfiguration message explicitly
being sent.
Reference is now made to FIG. 10a. In FIG. 10a, the process starts
at step 1010 and proceeds to step 1012 in which a check is made to
determine whether a precondition for long DRX exists. As will be
appreciated by those skilled in the art, such a precondition could
include one or more of: the DRX being configured for non real time
traffic, a lack of uplink and downlink traffic for the UE, low data
transmission to the UE, the position of the UE within a cell and
the likelihood of a transition occurring, among others. If, in step
1012, a determination is made that the precondition exists the
process proceeds to step 1020 in which a long DRX MAC CE is sent to
the UE.
Conversely, if the precondition in step 1012 does not exist, the
process proceeds to step 1030 in which the short DRX period is
maintained and the process proceeds back to step 1012.
From the UE perspective, reference is now made to FIG. 10b. The
process in FIG. 10b starts at step 1050 and proceeds to step 1052
in which a check is made to determine whether short DRX is
configured. If not, the process proceeds to step 1060 in which long
DRX is used.
Conversely, if it is determined in step 1052 that short DRX is
configured, the process proceeds to step 1070 in which a check is
made to determine whether a long DRX command has been received. As
will be appreciated, this long DRX command will be part of a MAC
command element and in one embodiment could be an explicit DRX
command indicating long DRX, which is distinct from the previously
configured DRX command. From step 1070, if a long DRX command has
been received, the process proceeds to step 1072 in which the UE
transitions to a long DRX. The process then proceeds to step 1074
in which a short DRX timer, if configured, is stopped. As will be
appreciated, this avoids having a short DRX start on the expiry of
the timer as a long DRX period is signaled.
From step 1074 the process proceeds to step 1080 and ends.
From step 1070, if a long DRX command has not been received the
process proceeds to step 1090 in which a short DRX timer is
started. The process then proceeds to step 1092 in which the UE
uses a short DRX period. The use of the short DRX timer allows the
UE to transition to long DRX after the timer has expired if no data
is received or sent during the timer period.
From step 1092 the process proceeds to step 1080 and ends.
The above provides the network with the ability to command the UE
to go directly to long DRX even when short DRX is configured. This
saves battery consumption and network resources when very low
traffic is observed by the eNB. The solution provides a more
efficient way to transition to long DRX than by sending RRC level
reconfiguration messages.
As will be appreciated by those skilled in the art, one issue with
the above solutions occurs in the case of a misinterpretation of an
ACK or a NACK. Specifically, the hybrid automatic repeat request
(HARQ), which is a variation of the ARQ error control method, does
not always properly demodulate an ACK or a NACK due to poor channel
conditions. Thus, in some situations, one can be interpreted as the
other. By having the DRX activation and deactivation occur in the
MAC-PDU header, an ACK to NACK or NACK to ACK misinterpretation
needs to be handled.
A possible solution to the above is the introduction of timer
threshold values before activating or deactivating DRX.
When the UE NACKs a MAC-PDU that has DRX header information, the UE
is unaware that it should adjust the DRX period. It will expect a
retransmission from the eNB. If a NACK to ACK misinterpretation
occurs, the eNB receives an ACK and it will not send a
retransmission and will change the DRX period. The UE waits for a
time to receive the retransmission. If the UE does not receive the
expected retransmission, the waiting time should be limited by an
upper threshold (TH-A) considering possible NACK to ACK
misinterpretations. If the UE does not receive a retransmission, it
should maintain its current DRX status. The eNB will expect an
exchange of information with the UE at the next DRX period. If the
UE does not respond, the eNB should revert to the previous DRX
period and attempt to "synchronize" with the UE.
Even when a UE ACKs a MAC-PDU, the UE needs to wait for
retransmission due to possible ACK to NACK misinterpretation or
possible ACK to DTX misinterpretation by the eNB. The waiting time
should be limited by an upper threshold (TH-B).
If the UE is missing data as indicated on the L1/L2 signaling
channel, assuming the eNB will retransmit at the next earliest
opportunity, the UE needs to check the L1/L2 signaling channel
within a certain duration (TH-C). Based on the various threshold
parameters above, the minimum time before DRX activation should
therefore be greater than (max(TH-A, TH-B)+TH-C). This threshold
value can be signaled either by system broadcast or RRC
signaling.
Various scenarios are considered herein:
DRX activation and ACK to NACK errors:
For an ACK to NACK misinterpretation or an ACK to a discontinuous
transmit (DTX) misinterpretation (i.e. the channel conditions are
so poor that the ACK appears as noise to the receiver), the
following occurs. The UE receives the DRX activation in the header
of the MAC-PDU and sends an ACK to the eNB. The eNB receives the
ACK but misinterprets it as a NACK or a DTX misinterpretation. This
results in the UE activating the DRX before the eNB, which may
result in the UE missing the retransmission of the MAC-PDU from the
eNB.
In the situations indicated above, an ACK to NACK or DTX
misinterpretation can be solved by the UE waiting for the timing
margin before activation of DRX. The margin can be based on the
normal time that it takes a retransmission to occur and weighted by
the average number of HARQ retransmissions to the UE that may be
experienced. The DRX activation may be indicated by the RRC
signaling or in the MAC-PDU header extension. When the UE
acknowledges the retransmission before the timing margin expires,
the UE will start the DRX at the system frame time considering
propagation delay and eNB processing time assuming that two
consecutive misinterpretations are very unlikely.
DRX Activation and NACK to ACK Errors:
Similarly, if the UE sends a NACK for a MAC-PDU, this could be
misinterpreted as an ACK by the eNB. In the case of DRX activation,
the eNB activates the DRX before the UE. If the eNB maintains the
CQI resource for the UE for a short period of time after activating
DRX, it will detect that the UE has not activated the DRX indicated
by checking the frequency of CQI report and it can signal the DRX
activation by L1/L2 control signaling. If the eNB releases the CQI
resource just after activating DRX and assigns it to another UE,
CQI reports from the two UE may collide. The eNB could use Time
Division Multiplexing or Code Division Multiplexing to avoid the
collision.
In the case that the RLC is operating in acknowledged mode (AM),
when a NACK to ACK misinterpretation occurs, recovery for DRX
synchronization between the eNB and the UE is established via the
normal RLC retransmission mechanism. This is because the RLC layer
in the transmitter will determine that the PDU is lost and
therefore instigate normal ARQ recovery by resending the original
data not received.
In the case that the RLC is operating in unacknowledged mode (UM
mode), no recovery mechanism exists. One solution is, in the HARQ,
the receiver sends a channel quality indicator (CQI). In continuous
reception, the channel quality indicator is repeated every 100
milliseconds, for example. Based on the CQI report, the transmitter
decides and indicates a coding rate, modulation scheme, and
Transport Block size. During active DRX, the eNB may expect a COI,
for example, every second. If the eNB gets this CQI at a different
rate (for example 300 milliseconds) it knows that the UE is not in
DRX and a correction can occur. For the deactivation DRX in a NACK
to ACK misinterpretation, the UE still thinks it is in DRX while
the eNB thinks it is in an active state. This can lead to missed
data; however, the next MAC-PDU an indication of DRX deactivation
can again occur.
Thus assuming that the CQI (channel quality indicator) reporting
will be aligned to the DRX length, the eNB will know if DRX
activation is completed in the UE by checking the frequency of CQI
reporting. If not completed, the eNB may use L1/L2 signaling or
send only a MAC-PDU header to correct the DRX activation or
reconfiguration.
Another recovery method can triggered when the eNB receives a
Timing Advance (TA) Request message from a UE that should be in
DRX. When the UE returns power to its transceiver and, hence,
emerges from the DRX state, it will often need to send control
(e.g. measurement reports) and other data messages the eNB. It is
important that the UE have the proper TA before sending these
messages so that the UE messages do not partially overlap with
messages from other UEs as they arrive at the eNB. Hence, after a
DRX cycle the UE will often send a TA Request on a random access
channel so that it can get the proper TA from the eNB. If the TA
request arrives at a point when the UE should be in DRX, the eNB
will know that the UE did not receive the last DRX activation or
modification properly. The eNB can then revert to the prior DRX
period for that UE and recover DRX-period synchronization.
DRX Deactivation and ACK to NACK Errors:
In the case of DRX deactivation or DRX length reconfiguration, an
ACK to NACK or DTX misinterpretation leads to the UE deactivating
the DRX before the eNB, which may require no special handling if
the UE acknowledges the normal retransmission from the eNB and the
eNB successfully received the ACK.
DRX Deactivation and NACK to ACK Errors:
In the case of DRX deactivation or DRX length reconfiguration, a
NACK to ACK misinterpretation results in the eNB deactivating the
DRX before the UE, which may result in the UE missing the new data
transmissions. The possible solution to this is that the eNB
indicates DRX deactivation on a MAC-PDU header extension of
subsequent MAC-PDUs. Assumptions are that consecutive
misinterpretations are very unlikely and that no DRX
reconfiguration is needed when only one MAC-PDU is needed to
transmit the new data which has arrived at the eNB.
DRX Automatic Incrementation
A further consideration is the incremental extension of the DRX.
Rules that dictate how the DRX period can be incremented or
decremented (e.g. by factors of two), in a preferred embodiment,
can be signaled during the radio bearer (RB) set up. The rules are
carried in the RRC RB set-up/reconfiguration or measurement control
messages to the UE. In this case, if no data is received after N
current DRX cycles, the eNB and the UE increase the DRX length to
the next larger value automatically. This eliminates the need for
signaling between the eNB and the UE to increase the DRX length and
therefore saves network and battery resources.
UE Request for DRX
Since the UE terminates all protocols from layer 1 to layer 7, the
UE may be able to determine if it can go into a longest DRX value
after receiving some specific data packets rather than waiting for
the network to increase the DRX value gradually. In this case,
however, it is required that the UE have the capability of
requesting DRX activation.
As will be appreciated by those in the art, the eNB is not very
intelligent when considering a UE higher layer or application
activities and thus would normally gradually increase the DRX.
However, the UE may know that the increase does not need to be
gradual in certain cases and can immediately go to a higher
value.
The eNB also signals if the UE may request for DRX activation via
the radio resource control radio bearer set-up or a reconfiguration
message. However, if the UE needs to inform the eNB of the
possibility of a rapid change, the user plane data is not always
available to piggyback a request for DRX from the UE. In a
preferred embodiment, L1/L2 signaling messages are used. The UE
sends a DRX request message to the eNB and the eNB replies with a
DRX grant message.
Various considerations may be taken into account by the UE besides
the application data flow characteristics in determining the proper
DRX period. The mobility and location within the cell, for example,
may be taken into account. If the UE is highly mobile or if it sees
good neighboring cells, the UE may choose to request a shorter DRX
period to prepare for a possible handover.
The eNB may also grant a shorter value than requested when it knows
that the UE is in a high-mobility state or the UE has already
missed handover opportunities, as described below. The eNB can also
consider how close the UE is to the cell's edge. If the UE is close
to a cell's edge, the eNB can reject or indicate a shorter time
value for the DRX.
If it is allowed by the eNB, the UE indicates a proposed value for
a DRX period in the optional field of uplink scheduling requests.
Even if the UE already has the uplink resources, the message is
used without the actual resource request part for the DRX
indication.
On the eNB, the eNB responds to the requests by indicating an
allowed value for the DRX. The activation time is also indicated if
the request for DRX is granted.
In some embodiments, the DRX request can be integrated in the UL
scheduling request and DRX grant can be integrated in the UL
scheduling grant.
The UE also considers its mobility and the likelihood of handover
when requesting DRX values, which can be based on the channel
quality measurement of the serving cell and its neighboring cells.
The UE may also increase measurement frequencies independently to
detect handover conditions more accurately, as described below. The
UE may consider its mobility status, whether high or low, which may
be based on positioning measurements, an accelerometer or the
filtering of L1 data.
Reference is now made to FIG. 4a. The process of FIG. 4 starts at
step 400 and proceeds to step 410 in which the UE receives
data.
The process then proceeds to step 412 in which the UE considers the
data and optionally considers other factors as described above.
Specifically, the UE may consider the mobility of the UE or the
signal strength of neighboring cells.
Based on the considerations of step 412, the process proceeds to
step 414 in which it requests a DRX in the L1/L2 uplink scheduling
request.
Reference is now made to FIG. 4b. The process then proceeds to step
416 in which the eNB receives the request.
The eNB, in step 418 considers the request and other optional
factors as described above. Specifically, the eNB may consider
whether the UE has missed a cell handover opportunity before or is
close to a cell boundary, or is highly mobile. In step 420 the eNB
decides whether to allow the request of step 414 based on the
factors in step 418. If yes, the process proceeds to step 430 in
which it signals that the request has been accepted. If no, the
process proceeds from step 420 to step 440 in which the eNB can
either reject the request completely or can suggest a shorter
duration for the DRX. Referring to FIG. 4a, the UE receives the
response from the eNB in step 442, and may acknowledge in step
444.
As will be appreciated by those skilled in the art, a long DRX may
lead to inaccurate handover decisions and executions by the UE.
When activating the DRX, the receiver will have less measurement
opportunity and thus the accuracy of the channel condition
estimation is degraded. Due to the measurement accuracy degradation
caused by DRX, the UE may miss a handover opportunity.
Based on the above, the eNB can reject the request or grant a
shortened DRX value if it knows that the UE is located close to a
cell edge. This decision can be based on the current timing
adjustment value assuming its available, the UE mobility status,
whether high or low, the number of handovers within a certain
period considering cell radius or the number of occasions that the
UE goes out of the serving cell or indeed knowledge regarding the
actual size of the cell (e.g. macro, micro or pico). These are all
factors that can be considered in step 418 of FIG. 4b.
Measurement Accuracy
A third factor for DRX in the LTE-ACTIVE state is the possibility
of missed handover opportunities. Since the UE receiver is turned
off during the DRX period, the measurement quality of serving and
neighboring cells is likely degraded compared to a continuous
measurement. This degradation may lead to premature handover or
missed handover opportunities, which should be avoided to the
maximum extent possible.
In order to reduce the number of premature handovers or missed
handover opportunities in DRX, in a preferred embodiment the UE is
allowed to have a shorter measurement cycle than the DRX cycle when
necessary. For example, if the channel quality of the serving cell
is lower than a threshold value A, the UE may start continuous
measurements or shorter measurement cycles to prepare for a
possible handover condition. If it turns out to be a false alarm,
i.e. if the channel quality obtained by the continuous measurement
is greater than a threshold value B, the UE can go back to the
measurement cycle equal to the DRX cycle. As will be appreciated by
those skilled in the art, the two threshold values represent better
channel conditions than a value that triggers handover so that a
sufficient level of accuracy is obtained when required to evaluate
handover conditions, thus missed handover opportunities can be
reduced.
In one embodiment of the present disclosure, the measurement
interval may be configured to be equal to a DRX interval divided by
N where N is an integer. This would be in the situation where the
mobile may be expecting a handover and/or there is high
mobility.
The network can configure the thresholds and the shorter
measurement cycles, and this can be signaled to the UE via
broadcast information or an RRC measurement control message. The
MAC-PDU header can indicate to the network the shortened DRX cycle
value once the UE has shortened the measurement cycle.
An example of the above is when there is an RRC connection or a
radio bearer is established. In this case, the eNB can indicate the
two channel quality values to which shorter DRX is started and
stopped respectively, and the ratio between measurement and DRX
cycles.
On the UE, the UE acts on the RRC signaling and starts or stops the
shorter measurement cycles according to the measurement of channel
quality compared to the threshold values.
Reference is made to FIG. 5. FIG. 5 illustrates various zones where
the UE may be situated including threshold values to indicate the
DRX cycle. In the first zone 510, the DRX cycle equals the
measurement cycle. The UE stays within this zone until it reaches a
threshold 520 in which it needs to start a shorter measurement
cycle.
The UE stays with the shorter measurement cycle until either the
signal degrades to indicate a handover condition 530 or if the
signal improves until it achieves an upper threshold 540, at which
point the DRX cycle and measurement cycle equal each other.
Preferably the eNB signals the following information in the radio
bearer set-up or in a measurement control message: A higher
threshold value used to lengthen the DRX cycle. This higher
threshold value indicates higher channel quality and/or signal
strength; A lower threshold value used to shorten the DRX cycle.
The lower threshold value indicates lower channel quality and/or
signal strength; Time-to-trigger associated with the higher
threshold value and the lower threshold value; and The handover
condition, such as the "best cell changed" and the measurement
cycle equals zero, indicating continuous measurement.
The diagram of FIG. 7 shows an example in which the channel quality
or signal power (as indicated in measurement reports) goes below
the lower threshold value (LTV) and then goes above the higher
threshold value (HTV) without uplink data. In this case, the
shortened measurement period is implemented between A and B,
whereas the DRX cycle equals the measurement cycle before A and
after B.
FIG. 8 shows an example in which the channel quality or signal
power (as indicated in measurement reports) goes below the lower
threshold value (LTV) and then goes above the higher threshold
value (HTV) with uplink (UL) data. In this case, the UE goes to a
short measurement cycle if the channel quality is below the lower
threshold value more than a certain duration (time to trigger). If
there is uplink data, the UE starts an initial UL access procedure
to obtain a UL resource grant by sending the scheduling request.
The scheduling request or the header of the uplink MAC-PDU could
indicate a request for a shorter DRX period. The eNB responds to
the request by sending scheduling grant message with a preferred
DRX value or the eNB could indicate a preferred DRX value in the
next downlink MAC-PDU. When the scheduling grant is received or the
downlink MAC-PDU is acknowledged, the eNB can start using the new
DRX value. The figure then shows the channel quality or signal
power (as indicated in measurement reports) goes above a higher
threshold value for a certain duration (time to trigger). The UE
indicates a request for a longer DRX value in the scheduling
request or in the header of the MAC-PDU if UL data is available.
The eNB responds to the request by sending a scheduling grant
message with a preferred DRX value and an indication to start the
automatic mode or the eNB could indicate a preferred DRX value with
an indication to start the automatic mode in the header of the next
downlink MAC-PDU. When the scheduling grant is received or the
downlink MAC-PDU is acknowledged, automatic mode is started with
the initial DRX value specified by the eNB. If no data is available
then the UE needs to send a L1/L2 control message to request the
automatic increment of DRX.
The example of FIG. 9 shows a handover condition trigger. In this
case, the signal is gradually degrading until it proceeds below a
lower threshold value for a certain time to trigger, at which
point, the UE starts using a shorter measurement. The UE then sees
the handover condition for a certain duration (time to trigger). At
this moment the UE initiates the UL access procedure and transmits
a scheduling request in order to obtain UL resources for the
measurement report message. DRX value of zero or a request for
going back to continuous reception mode can be indicated in the
scheduling request or in the MAC-PDU carrying the measurement
report message. The eNB responds to the request by sending
scheduling grant message with the preferred DRX value of zero or
the eNB could indicate a preferred DRX value of zero in the next
downlink MAC-PDU. When the scheduling grant is received or the
downlink MAC-PDU is acknowledged, both sides deactivate DRX. On
handover, the UE receives a handover command and obtains downlink
synchronization to the target cell. The UE then indicates the
channel quality and/or signal strength of the target cell in the
handover complete response. The eNB can then evaluate when it is
safe to activate DRX. If so the eNB indicates DRX activation in the
downlink (DL) MAC-PDU header or L1/L2 control signaling.
In the above paragraphs, the requests made by the UE for a shorter
or longer DRX period or the DRX value itself are in the scheduling
request or the header of the uplink MAC header. The eNB responds to
the UE, by specifying the preferred DRX period with an indication
if an automatic DRX increase/decrease rule can be applied, within
the scheduling grant or downlink MAC-PDU header.
In another embodiment, the scheduling request indicates the cause
of uplink access. For example, suppose that during a DRX period of
2.56 seconds a VOIP call is originated. In order for the network to
respond the VOIP call setup promptly, the UE sends the scheduling
request with a cause of uplink access, e.g. call setup. The eNB
replies to the request by sending a scheduling grant indicating a
DRX value of zero (the preferred DRX value).
Detection and Handling of Very Late Handover
In order to utilize the DRX in the LTE_ACTIVE state, a standardized
criterion for the UE to determine if a handover opportunity is
missed is preferable. If such a condition is satisfied, the UE
should establish a connection to a neighboring cell rather than the
serving cell. As will be appreciated by those skilled in the art,
in the LTE infrastructure, only network based handover procedures
apply and there are no UE based procedures such as cell reselection
as used in UMTS.
If, in a preferred embodiment, the channel quality of the serving
cell is less than a neighboring cell by a threshold value C for a
certain time duration T, the UE is required to connect itself to
the neighboring cell on the target eNB. The value C and T can be
signaled by system broadcast information or RRC signaling.
The process for switching to the target eNB includes the steps of:
1. Start UL initial access procedure to obtain a timing advance
value for the target cell and uplink resources for the subsequent
control messages; 2. Transmit a reconnect request to the target eNB
with the current RNTI (radio network temporary identifier) and
previous cell ID; 3. The target eNB contacts the serving eNB in
order to obtain the UE context and downlink data needs to be
transferred. The target eNB also connects itself to the access
gateway and removes the serving eNB from the aGW; and 4. The target
eNB transmits a reconnect response to the UE with a new RNTI and
uplink grant.
An optional component includes a status message to be carried over
the reconnect request and response so that the amount of data
transferred between the target and serving eNB and between the
target and serving eNB and the air interface with the UE can be
minimized.
Optimizations include the reconnect request in step 2 above to be
sent with a status report showing the PDCP (packet data convergence
protocol) SDU (service data unit) sequence numbers which the UE has
received successfully. This information helps to reduce the amount
of downlink user data to be transferred from the serving to the
target eNB and over the air to the UE. Since the RLC is likely
reset in the procedure, PDCP SDU sequence numbers need to be
used.
Likewise, the reconnect response can be sent with a status report
showing PDCP SDU sequence numbers which the serving eNB receives
successfully so that the UE can retransmit data that was
missed.
Further, if the target eNB finds that there is no data to be
transferred from the serving eNB and from the aGW, the reconnect
response indicates DRX activation.
The above is illustrated in FIG. 6a in which, in step 612, the UE
obtains a timing advance value for the target cell and uplink
resources for the subsequent control messages. The process then
proceeds to step 614 in which the UE transmits a reconnect request
to the target eNB with the current RNTI and cell ID. The UE then
waits for and acknowledges a response from the target eNB in step
650.
Referring to FIG. 6b, the target eNB receives the request at step
615 and then proceeds to step 616 in which the target eNB contacts
the serving eNB in order to obtain the UE context.
In step 618, the target eNB transmits a reconnect response to the
UE with the new RNTI and uplink grant.
The above can be implemented on any UE. Such UEs include, but are
not limited to, personal digital assistants, cellular telephones,
wireless data devices, among others.
The embodiments described herein are examples of structures,
systems or methods having elements corresponding to elements of the
techniques of this application. This written description may enable
those skilled in the art to make and use embodiments having
alternative elements that likewise correspond to the elements of
the techniques of this application. The intended scope of the
techniques of this application thus includes other structures,
systems or methods that do not differ from the techniques of this
application as described herein, and further includes other
structures, systems or methods with insubstantial differences from
the techniques of this application as described herein.
* * * * *